リン酸-酸化鉄-PVB系さび安定化処理膜のイオン選択透過性と安定さび生成機構

概要

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Despite thier thinness, films applied to weathering steel using a phosphoric acid-iron oxide-PVB rust stabilization surface treatment act to inhibit rust formation and to suppress“rust outflow”. The following conclusions were reached in a study of the reasons for the treatments effectiveness.(1) The treatment film exhibits cation selectivity in a NaCl solution and KCl solution and anion selectivity in a FeCl2 solution and FeCl3 solution. Accordingly, by acting to suppress the diffusion of Cl-, Fe2+ and Fe3+, the film inhibits corrosion of the iron matrix and the occurrence of“rust out-flow”.(2) Examination of artificial rust (amorphous rust) revealed that a compound layer comprised of the treatment film and rust acts as a bipolar film with both cation selectivity and anion selectivity. The compound layer also suppresses the diffusion of Cl-, Fe2+ and Fe3+.(3) Because the treatment film has a relatively high permeability for moisture and oxygen, PO43- and other phosphate ions easily flow out of the film during prolonged atmospheric exposure. It is thought that PO43-, in addition to imparting ion selectivity to the rust, contributes, together with the alloying elements, to rust stabilization in weathering steel.
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